Background: Chemoprevention of breast cancer, which continues to be a leading cause of cancer-related mortality among women globally, is feasible as exemplified by clinical success of selective estrogen receptor (ER) modulators (SERM) and aromatase inhibitors. However, the SERMs are ineffective against ER- negative breast cancers and have serious side effects. The aromatase inhibitors appear promising for prevention of breast cancer in high-risk postmenopausal women. Even though the safety of aromatase inhibitors in a preventative setting is still under study, their use raises concerns for increased risk of cardiovascular events and bone fracture. Therefore, a non-toxic preventive intervention effective against both ER-positive and ER-negative breast cancers is highly desirable. In the previous funding period, we demonstrated that a small-molecule (withaferin A; hereafter abbreviated as WA) derived from an Ayurvedic medicine plant (Withania somnifera) not only prevents ER-negative mammary cancer development in a clinically-relevant transgenic mouse model (MMTV-neu) without any toxicity, but also inhibits ER- ? expression and activity at pharmacologic doses in cultured human breast cancer cells. At the molecular level, we have identified novel mechanistic targets of WA, including tubulin (downregulation of ?- and ?- tubulin as well as covalent modification of Cys-303 of ? -tubulin in MCF-7 cells) and complex III of the mitochondrial electron transport chain (activity inhibition), potentially contributing to its chemopreventive activity by eliciting mitotic arrest and reactive oxygen species (ROS)-dependent apoptotic cell death. These effects of WA are observed in mammary cancer cells representing major subtypes, including ER-positive, ER-negative, and triple-negative breast cancer. More importantly, normal mammary epithelial cells are significantly more resistant to tubulin and complex III targeting by WA compared with breast cancer cells. Despite these exciting mechanistic findings, the functional significance of down-regulation or Cys-303 modification of tubulin in growth arrest by WA is still unclear. Likewise, the molecular basis for WA-mediated inhibition of complex III activity remains elusive. Hypothesis: The present renewal application logically builds upon these novel and largely published observations to test an exciting hypothesis that WA administration prevents both ER-positive and ER-negative (already shown in the previous funding period) breast cancers in relevant animal models in association with post-translational modifications of tubulin(s) and complex III subunit(s) leading to mitotic arrest and ultimately apoptotic cell death selectively in cancerous cells. Specific Aims: The proposed research utilizes relevant cellular and in vivo animal models of breast cancer and cutting-edge technologies to: (1) determine the efficacy of dietary WA administration for prevention of ER-positive mammary cancer in a well-established rat model of chemically-induced (N-methyl- N-nitrosourea) breast cancer; (2) study the functional significance of downregulation and post-translational modification of tubulins in mitotic arrest by WA; and (3) determine the molecular mechanism by which WA inhibits complex III activity. Translational Impact: The progress in the previous funding period exceeded our own expectations as evidenced by completion and publication of the proposed work as well as identification of novel mechanistic targets of WA. The translational impact of the studies proposed in this renewal application is ultimately realized by: (a) rational design of a pilot biomarker-driven trial in a neoadjuvant window setting, which is beyond the scope of this application because clinical trial design without a full appreciation of the molecular pharmacology of WA is premature; (b) identification of mechanistic biomarker(s) predictive of WA exposure, and possibly response, which is critical for its clinical development because primary cancer incidence is too demanding of an end point; and (c) identification of a non-toxic regimen for targeting tubulin/microtubule network selectively in cancer cells as currently available anti-mitotics (e.g., taxanes) have side effects. In this era of targeted therapies and personalized medicine, the tubulin/microtubule network still remains an attractive therapeutic target for breast and other cancers.